The present invention relates to a method of forming an oxide film on a semiconductor layer on a surface of a substrate in the production of a semiconductor device.
For example, in producing a MOS type silicon semiconductor device on the basis of a silicon semiconductor substrate, it is required to form a gate oxide film composed of a silicon oxide film (SiO.sub.2 film) on a surface of the silicon semiconductor substrate. In the production of a thin film transistor (TFT), it is also required to form a gate oxide film composed of a silicon oxide film (SiO.sub.2 film) on a surface of a silicon layer formed on an insulating substrate. It is no exaggeration to say that the reliability of semiconductor devices depends upon these silicon oxide films. The silicon oxide films are therefore constantly required to have high dielectric breakdown durability and long-term reliability. In a MOS type silicon semiconductor device, a silicon oxide film is used for forming a device-isolating film and an insulating interlayer in addition to the gate oxide film.
The method of forming the above silicon oxide films includes a thermal oxidation method, a vapor phase growth method and a sputtering method. For forming silicon oxide films having a very small thickness of several nanometers to ten and several nanometers for use as a gate oxide film, a thermal oxidation method is used in view of excellent interface characteristics and easiness in controlling a film thickness.
As a thermal oxidation method, there has been hitherto used a method in which high-purity water vapor is introduced into a process chamber made of a quartz furnace core tube heated with a cylindrical resistance heater to thermally oxidize a surface of a silicon semiconductor substrate (to be referred to as "wet oxidation method" hereinafter). The wet oxidation method is said to be feasible for forming an oxide film having excellent electric reliability over an oxide film formed by an oxidation method using a high-purity dry oxygen gas (to be referred to as "dry oxidation method" hereinafter). In the wet oxidation method, generally, a hydrogen gas is mixed with an oxygen gas at a high temperature and water vapor generated by the combustion of the hydrogen gas is used. Such a wet oxidation method is called a hydrogen combustion oxidation method or a pyrogenic oxidation method. An oxide film formed by the wet oxidation method will be sometimes called a wet oxide film, and an oxide film formed by the dry oxidation method will be sometimes called a dry oxide film.
The above process chamber with a resistance heater includes a horizontal type process chamber which is a horizontally maintained process chamber made of a quartz furnace core tube and a vertical type process chamber which is a vertically maintained process chamber made of a quartz furnace core tube. Meanwhile, a natural oxide film is formed due to inclusion (inflow) of atmosphere during the transfer of a silicon semiconductor substrate into a process chamber, and the influence of the natural oxide film is no longer negligible with a decrease in the thickness of an oxide film. The natural oxide film contains impurities from atmosphere and dry oxide film component. When the amount ratio of the natural oxide film contained in the oxide film increases with a decrease in the oxide film, the oxide film formed by the wet oxidation method deteriorates in electric characteristics. Therefore, the vertical type process chamber constitutes the main stream at present, since it makes it easy to prevent the inclusion (inflow) of atmosphere.
If a silicon semiconductor substrate is transferred into the process chamber in a state where the inner atmosphere in the process chamber is an inert gas atmosphere or a vacuum atmosphere, the formation of a natural oxide film can be suppressed. Since, however, the natural oxide film works as a protective oxide film on one hand, it may cause a decrease in the electric characteristics and reliability of a semiconductor device if the formation of the natural oxide film is simply suppressed.
When a MOS type silicon semiconductor device is produced on the basis of a silicon semiconductor substrate, generally, the silicon semiconductor substrate is subjected to RCA cleaning before an oxide film is formed. That is, the surface of the silicon semiconductor substrate is cleaned with an NH.sub.4 OH/H.sub.2 O.sub.2 aqueous solution and further cleaned with an HCl/H.sub.2 O.sub.2 aqueous solution to remove fine particles and metal impurities from its surface. When the RCA cleaning is carried out, the surface of the silicon semiconductor substrate reacts with the cleaning solutions, to form a silicon oxide film having a thickness of approximately 0.5 nm to 1 nm. The thus-formed silicon oxide film has a non-uniform thickness and has a residual cleaning solution content. The silicon semiconductor substrate is therefore immersed in a hydrofluoric acid aqueous solution to remove the above silicon oxide film, and further, a chemical component is removed with pure water. As a result, the silicon semiconductor substrate eventually has a surface most of which is terminated with hydrogen and a very small part of which is terminated with fluorine. In the present specification, obtaining a silicon semiconductor substrate surface most of which is terminated with hydrogen and a very small part of which is terminated with fluorine by the above step will be described as exposing a surface of a silicon semiconductor substrate. Then, an oxide film is formed on the surface of the above silicon semiconductor substrate.
Meanwhile, when a silicon semiconductor substrate is exposed to a high-temperature inert gas or vacuum atmosphere, the silicon semiconductor substrate is caused to have a roughened surface due to the elimination of silicon atoms and fluorine atoms from the silicon semiconductor substrate and a nitriding reaction in the silicon semiconductor substrate, so that the oxide film is poor in an electric insulation property and reliability. Nevertheless, the natural oxide film works to prevent the above surface roughening. For reducing the natural oxide film, therefore, it is required to attain the prevention of roughening of an interface between the oxide film and the silicon semiconductor substrate at the same time.
As a method for overcoming the above problem, for example, there can be a method in which a silicon semiconductor substrate is transferred into the process chamber with flowing oxygen gas at a very small rate. This method will be explained with reference to FIG. 10 showing a schematic cross-sectional view of an oxide film forming apparatus having a conventional vertical type process chamber (oxidation furnace).
Silicon semiconductor substrates on each of which a silicon oxide film is to be formed are transferred onto a quartz boat 24 disposed in a substrate transfer portion 20 through a door (not shown). In this occasion, a shutter 15 is closed, and a process chamber 10 is maintained to internally have a nitrogen gas atmosphere (temperature: 700.degree. C.) containing 0.1% by volume of oxygen gas.
In this state, an inert gas (for example, nitrogen gas) is introduced into the substrate transfer portion 20 at a flow rate of 300 SLM from a gas inlet port 21 and discharged out of a gas outlet port 22, whereby the atmosphere in the substrate transfer portion 20 can be changed into an inert gas atmosphere. And, an oxygen concentration meter (not shown) is used to measure a concentration of residual oxygen gas in the substrate transfer port 20. When the oxygen gas concentration is found to be 100 ppm or less, the shutter 15 is opened, and the quartz boat 24 is moved upward with an elevator mechanism 23 to transfer the silicon semiconductor substrates into the process chamber 10.
After the silicon semiconductor substrates are transferred into the process chamber 10 in the above manner, oxygen gas and hydrogen gas are introduced into a combustion chamber 30 through pipings 32 and 33 and mixed in the combustion chamber 30 to combust the hydrogen gas for generating water vapor. The water vapor is introduced into the process chamber 10 through a piping 31, a gas line 11 and a gas inlet port 12. Then, thermal oxidation is initiated as described above. In this case, the formation of a natural oxide film caused by including atmosphere (inflow of atmosphere) can be prevented, and further, the above protective oxide film is formed on the surface of the silicon semiconductor substrate by a very small amount of oxygen gas, so that the surface roughening of the silicon semiconductor substrate can be prevented.
As other means, after the silicon semiconductor substrates are transferred into the process chamber maintained at a temperature of as low as 300.degree. C., a thin silicon oxide film is formed on the surface of the silicon semiconductor substrate by a dry oxygen gas, then, and the atmosphere in the process chamber is changed to an argon atmosphere, followed by temperature-increase up to 900.degree. C. Then, thermal oxidation is carried out. In this case, it is said that the oxide film formed at a low temperature works as a protective oxide film, so that the surface roughening of the silicon semiconductor substrate can be prevented (see, for example, Proceeding of No. 25 Ultra-Clean Technology Workshop, pages 3 and 4, sponsored by USC Semiconductor Substrate Technology Researchers Meeting).
However, the above method uses, as a protective oxide film, a dry oxide film formed by oxygen gas, which means that a dry oxide film having a certain thickness is formed prior to the formation of a wet oxide film at a high temperature. When the above method is applied, for example, to the process of forming a gate oxide film, the amount ratio of the dry oxide film component increases with a decrease in the thickness of the gate oxide film, and as a result, excellent properties of the wet oxide film decreases.